Magnetic resonance is an immensely useful analytical technique that can be applied to electrons, to nuclei, or sometimes, both simultaneously. Electron spin resonance (ESR) a technique which is also sometimes referred to as electron paramagnetic resonance (EPR) and its nuclear analog, nuclear magnetic resonance (NMR) are among the most powerful and widely utilized analytical tools of the past sixty years for applications in medicine, chemistry, biology, solid state electronics, archaeology, and many other fields, far too numerous to list.
ESR is applied in areas which are as mundane as evaluating the shelf life of beer and to areas as exotic as estimating the age of exceptionally ancient artifacts. ESR is utilized in the pharmaceutical industry to study the way certain drugs attack disease and can be utilized to understand the nature of disease at a fundamental molecular scale. An example of an ailment under study via ESR is mad cow disease. ESR is used in the electronics industry to understand fundamental materials based limitations in the performance of integrated circuits. ESR, in the form of electrically detected magnetic resonance (EDMR), may have great potential in the future in quantum computing.
Briefly, in electron spin resonance and in other types of magnetic resonance, energy is absorbed by a spin (that of an electron in ESR and a nucleus in NMR) when a particular relationship exists between a large applied magnetic field vector, the spin center under observation, and the frequency of electromagnetic radiation (radio frequency or microwave frequency) applied to the sample under observation. The relationship conveys a great deal of information about the physical and chemical nature of the spin's atomic surroundings. Depending upon the specific application, this information can help evaluate the potential of a drug in the treatment of disease or identify physical imperfections that limit the performance of integrated circuits, or determine the age of an ancient artifact. Many applications are possible.
Nearly all scientific measurements involve some sort of electrical signal which encodes useful information. These electrical signals consist of a component which carries the physical, chemical, or biological information of interest and a noise component. Noise is the undesirable component of the total signal. The ratio of signal to noise is a generally a meaningful measure of the quality of the scientific measurement. If the signal to noise ratio falls below a certain value, the measurement becomes meaningless. The signal to noise ratio is typically a function of the time involved in making the measurement. When the noise is random in nature, which is often the case, the signal to noise ratio can be improved by increasing the time involved in measurement. This is often done by signal averaging, that is, repeating a (repeatable) measurement over and over, then averaging the measurements. In conventional signal averaging, the signal to noise ratio improves as the square root of the number of repetitions.
EDMR typically involves spin dependent recombination (SDR). EDMR in general and SDR in particular are electron spin resonance (ESR) techniques in which a spin dependent change in current provides a very sensitive measurement of paramagnetic defects. Without special application of digital signal processing techniques, EDMR measurements involving SDR are about 7 orders of magnitude more sensitive than conventional ESR. The techniques are therefore particularly useful in studies of imperfections in the semiconductor devices utilized in integrated circuits. In such devices, the dimensions are quite small and can have very low defect densities. SDR detected EDMR can be utilized in fully processed devices such as metal oxide semiconductor field effect transistors (MOSFETs), bipolar junction transistors (BJTs), and diodes. With some additional improvements, the technique's very high sensitivity may make it potentially useful for single spin detection and quantum computing. However, the sensitivity EDMR is not currently high enough to detect a single spin in the presence of the noise encountered with present day EDMR spectrometers in a reasonable amount of time.
Continuous wave magnetic resonance typically utilizes a sinusoidal modulation of the applied magnetic field, thereby encoding the signal in a sinusoid. The amplitude of the modulated signal is a measure of the magnetic resonance signal, in this case, an EDMR detected ESR signal. ESR measurements in general and, in the specific case utilized herein, EDMR, can provide a measure of the number of paramagnetic defects within the sample under study as well as the means to identify the physical nature of these defects. Magnetic resonance in general can provide a very broad range information about physical and chemical structure. In continuous wave magnetic resonance, a lock-in amplifier (LIA) is generally utilized to demodulate the amplitude modulated magnetic resonance signal to DC, thus exploiting the sensitivity enhancement available from the phase and frequency detection. This widely used method effectively attenuates much of the noise in the magnetic resonance measurement. In the specific EDMR detected ESR example utilized here, much of the noise is associated with the 1/f noise typically observed with a DC current produced by the transistor.
Although lock-in detection is quite powerful, it is often insufficient to achieve a reasonable signal-to-noise ratio (SNR), so signal averaging is also often utilized in magnetic resonance. In cases in which the single measurement SNR is particularly low, extensive signal averaging may be required to glean useful information from the magnetic resonance measurements. In our demonstration we utilize ESR spectra detected through EDMR in transistors.
Though work has been performed to remove noise observed in related fields via software such as nuclear magnetic resonance (NMR), not much has been done in any area of ESR including EDMR.